CS 201 A Whirlwind Tour of C Programming Gerson Robboy Portland State University

A brute simple Makefile all: sd test1 t1check test2 sd: sd.c cc -g -o sd sd.c test1: test1.c cc -o test1 test1.c test2: test2.c cc -o test2 test2.c t1check: t1check.c cc -o t1check t1check.c clean: rm sd test1 t1check test2 t1sort

Some things about Makefiles Call it makefile or Makefile (big or little M) The “make” utility will use that by default The first rule in the Makefile is used by default if you just say “make” with no arguments The second line of each rule (the command) must start with a tab, not spaces!

A slightly more complex makefile CC = gcc CFLAGS = -Wall -O2 LIBS = -lm OBJS = driver.o kernels.o fcyc.o clock.o all: driver driver: $(OBJS) config.h defs.h fcyc.h $(CC) $(CFLAGS) $(OBJS) $(LIBS) -o driver driver.o: driver.c defs.h kernels.o: kernels.c ... h fcyc.o: fcyc.c clock.o: clock.c

A brute simple shell script test1 sd t1file t1sort t1check echo test2 sd t2file t1sort t1check echo A plain text file containing commands Known as a batch file in some O. S.’s Make sure the file has “executable” permission Invoke it by name, just like a binary program

argc and argv main has two arguments: main(int argc, char *argv[]) argc tells how many command line arguments there are, including the command itself argv is a pointer to an array of pointers to characters Example: find . –print argc = 3 argv[0] = “find” argv[1] = “.” argv[2] = “-print”

The C preprocessor bass> gcc -O2 -v -o p m.c a.c cpp [args] m.c /tmp/cca07630.i cc1 /tmp/cca07630.i m.c -O2 [args] -o /tmp/cca07630.s as [args] -o /tmp/cca076301.o /tmp/cca07630.s <similar process for a.c> ld -o p [system obj files] /tmp/cca076301.o /tmp/cca076302.o bass> cc or gcc, the compiler driver, invokes a bunch of things The first one is cpp, the preprocessor After that is cc1, the translator cpp converts the C source file to another C source file expands #defines, #includes, etc.

Things you can tell the preprocessor Included files: #include <foo.h> #include “bar.h” Defined constants: #define MAXVAL 40000000 Macros: #define MIN(x,y) ((x)<(y) ? (x):(y)) #define RIDX(i, j, n) ((i) * (n) + (j)) Conditional compilation: #ifdef … #if defined( … ) #endif

What do you use conditional compilation for? Debug print statements By defining or undefining one constant, you can include or exclude many scattered pieces of code Code you think you may need again #ifdefs are more readable than commenting code out Portability To multiple operating systems To multiple processors Compilers have “built in” constants defined #if defined(__i386__) || defined(WIN32) || … To different compilers for the same processor

Pointers Pointers can point to any data type, including structures pixel foo[32000]; // An array of pixels // p is a pointer to a pixel, initialized to foo pixel *p = foo; &(foo[99]) is the same thing as (p+99) You can use subscripts with pointers Incrementing a pointer adds the increment times the size of the data type that it points to. foo[99] is the same as *(p+99) or p[99] foo[0].blue is the same thing as p->blue foo[99].red is the same thing as (p+99)->red

Pointer example Following a linked list struct zilch listhead; struct zilch *p = listhead; while (p != NULL) { ... // Do stuff to *p p = p->next; }

What’s the problem with this? int zarray[34000]; char *zp = zarray; int i; for(i=0; i<34000; i++){ *zp++ = i; }

C pointer declarations int *p p is a pointer to int int *p[13] p is an array[13] of pointer to int int *(p[13]) p is an array[13] of pointer to int int **p p is a pointer to a pointer to an int int (*p)[13] p is a pointer to an array[13] of int int *f() f is a function returning a pointer to int int (*f)() f is a pointer to a function returning int int (*(*f())[13])() f is a function returning ptr to an array[13] of pointers to functions returning int int (*(*x[3])())[5] x is an array[3] of pointers to functions returning pointers to array[5] of ints